rolas27

2017 A6 43k miles. I get this strange knocking sound from the top back of the engine. It looks like the sound is coming from the supercharger.

Car runs great otherwise, no loss of power, no errors in VCDS except “Oxygen Sensor Failed or Incomplete” in “Readiness”.

This sound only occurs when the engine is warm and on idle. It disappears when I accelerate.

Car slightly vibrates on idle and RPMs jump in range of 690-710. Did anyone deal with this? Is this normal?


Video:

xanmato

Should be normal. What you are hearing is either the vacuum pump, or secondary air pump.

Edit: Actually I take my previous statement back, as I was able to listen to the video on some speakers.

My 13' A6 had this sound for 20k+ miles before I traded it in. Same conditions as described, though when shifting into drive or reverse the sound would go away. In park or neutral it would come back. My 16' A6 has this sound but very faint or not at all sometimes. My original guess was something to do with the torque converter as the sound went away when shifting into a gear. I also watched a mechanics video of the exact same issue and they cannot find the source either, guessing that it could be a noisy oil pump. That vehicle was said to be fine after years as the same mechanic serviced it. I am still thinking it's some kind of pump noise, definitely not rod knock as this sound rhythm is 1:1 and not 2:1, and definitely not a metal tapping sound.

My own scoping has come back with inconclusive results, as I don't want to tear apart the engine looking for something that doesn't appear to be doing any damage at the moment.

Acuransx

Wow, what timing. As of last Thursday/Friday I noticed same in my /15 3.0T. Almost exactly same as the video that the OP put up, maybe not quite as loud. The car is just a shade over 5 years old and has 46,000 miles. No warranty, owned since new and babied -- all maintenance done, oil changes every 5,000 miles. Runs perfect, other than the subtle knock. No CEL or other warning lights and no other funny noises.

Last Thurs or Friday it was a beautiful day here in MI and I drove home with the sunroof and windows open which I almost never do. I pulled into the garage and put the thing in park, closed roof and all windows before I shut her down. Just as I'm about to hit the kill switch I hear knocking -- subtle but not imagined. I put the thing in drive and no knock. I backed up a bit and popped hood and heard that noise -- lots of noise under hood (this is not exactly a quiet motor with those DI injectors). I spent quite a bit of time listening and trying to figure out where the noise was loudest. Same as the OP -- seems to be coming from back of the S/C. The noise is highly audible if I have the car running in park and get down on the ground just aft of the driver's side front wheel close to the driver's door. For sure a knocking sound. When I'm in the car idling in neutral or park the knock is not audible if the climate control is operating.

I've spent a few days listening for this and determined that car does not make this noise until it is warmed up. In addition, if I listen very closely as I rev the car gently from normal idle speed once it is warm up to say 1200/1300 rpm the knocking does seem to speed up and slow down as engine revs up and falls back to idle speed.

To say I'm totally freaked out by this is an understatement. Maybe this has been going on for some time but it is very disconcerting and concerns me. I dropped by the dealer and went for a ride with the lead tech. He heard it and thought maybe a purge valve at back of the S/C but after he took the plastic cover off he didn't think it was that. He was going to listen to a few C7 3.0T A6s as they come into the shop and get back to me -- also said would be happy to take it into the shop on a no charge basis to look into this.

As I said, I'm very nervous about this. I do like the car but it hasn't exactly been a model of perfection and I do worry about expensive issues that might arise without any warranty coverage. I'm at the point where I need tires, brakes and now the flex pipe is beginning to show signs of needing attention, though that job is likely something that can wait. I sure want to get to the bottom of this new issue before I spend the dough for tires and brakes. Not happy that the OP has this issue too but glad that I'm not the only one and it will be good to hear what my AW compadres have to say about this.

Best,
Jeff

rolas27

I'm glad I'm not the only one with this issue. Car runs fine with it though, so I don't think it's a major failure of the engine. I'm taking my car to a mechanic next week. I sent him a video and he told me that it could be a loose flywheel. But he's not certain because it's very hard to do diagnostics just by looking at the video I still hope it's some kind of a valve since the sound shows up only when the engine is warmed up and that it would not be an expensive fix.

Acuransx, keep us updated in case you'll find out what is causing that knocking

Acuransx

Will do. It will be interesting to see how many others have a similar noise. I fear that this may be one of those things that isn't capable of diagnosis without tearing into the engine/trans/peripherals and/or throwing parts at the car, which are rabbit holes I'm not going to go down.

rolas27

Just got back from the mechanic. From the first look he told me that it sounds like a noisy oil pump. He suggested me to change the oil to a thicker one - 5w40 since I’m using 5w30 right now. Also they found some misfires in 4th and 6th cylinders so they will replace the spark plugs next week and inspect the knocking noise further.

Acuransx

I just got back from the dealer. Car was up in the air and belly pan off. Tech listened to engine with stethoscope and did a general inspection. His conclusion was that noise is not a metallic knocking and does not believe anything deep within engine. He said he recalls hearing this noise before when doing trans service on some A6s. No conclusive determination of where/what exactly this noise is. He felt I should not have any major problems with the engine based on service history and what he heard -- hoping he is correct. I will make sure it is documented in dealer's system so there is something to talk about with Audi if the worst happens.

Going forward, I'm just going to drive it and ignore the noise which is only evident at idle in neutral/park and not heard when I'm in the car with the climate control operating. Car will get its 55K major service spring/summer next year and assuming no catastrophic issues in the interim tech will take another look at this issue then. I am also going to have the car at a well-respected indy in the near future to look at the flex pipe which is starting to go and while there will ask that they offer an opinion on the knock. As I said in a prior post -- not going to go down the rabbit hole of spending inordinate amounts of time/money trying to figure this out -- if anything of any real substance it will make itself known as time goes by. Not thrilled about this but I'm going to roll the dice that the expert advice I've been given is correct and that this engine with all of 46K miles is not going to have a major issue.

I'll update this post periodically.

Best,
Jeff

Acuransx

Update -- I had lunch with a friend today who has a /13 A6 3.0T. I gave her car a good listen. There is a subtle knocking sound similar to my car but not as loud -- same location -- back of the S/C -- isn't the throttle body back there -- maybe coming from that? I have a feeling that many of the 3.0Ts make this noise -- some louder and some less so. I'm going to heed the dealer's lead tech's recommendation and not worry about this. Per my post above, I'll report back if anything untoward happens and in any event towards end of year when I expect to have accumulated a few more miles.

Best,
Jeff

BizAsUz

I have the same knocking sound on my 2016 A6.. this I assume is normal (while in park). once driving no knocking sounds are evident.

Acuransx

I'm not sure I'd call this a normal condition. My friend's A6 has the noise but not as loud as in my car. While I was at the dealer a fellow showed up in a /16 A6 3.0T (mine is a /15) and I asked him to pop his hood -- I could not hear this noise in his car.

Whether all cars have the noise or not I don't know. Of those that have the noise some are louder than others. I'm told by the dealer's lead tech who is very good and would not intentionally steer me wrong that he's not sure what this noise is/where exactly it is coming from but he is sure that it isn't deep within the engine and that it is not a rod knock or a bearing issue. His advice was to drive the car and ignore this which is what I will do. However, it would be nice to know exactly what is causing this noise.

Best,
Jeff